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4 Answers
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Despite the similarity, I've never thought of these as meaning the same thing because their usage is so different:

わざわざ means that someone has gone out of their way to do something.

せっかく means that someone has made an effort, or some unexpected opportunity has come up, and it would be a shame to waste it. Mostly it is used positively in the sense of grasping the opportunity, but sometimes it is used in turning down the effort.

Sekkaku is used in the expressions sekkaku... no ni, sekkaku ... dakara, sekkaku no..., and expresses a feeling that since an effort has been made to do something, it should not be wasted. In the form sekkaku no ... da ga or sekkaku de wa aru ga, it is used to refuse the other person's request or proposal. Also, in expressions like sekkaku no yasumi ni ame ga furu ('just when we had a holiday it rained') it expresses regret at a situation where a rare opportunity is wasted. Also written 折角.

Wazawaza describes a situation where, although there was no necessity to go to the trouble, a person has specially gone out of their way to do something, and not merely in passing. In particular, it is often used where something is done for another person out of concern or care for that person. It is also used in cases where someone is going out of their way to do something that they don't need to.

(Please forgive the clunky translation. The explanations are full of Japanese terms that don't go easily or directly into English).

In 1) the emphasis is on appreciation of an effort or opportunity and regret at the possibility that it will be wasted. In 2), the emphasis is on the fact that someone has gone out of their way to do something, usually (but not always) with appreciation of that action.

It is interesting that you've managed to come up with two examples where わざわざ and せっかく are similar in meaning and usage!

わざわざ来てくれたからご飯をおごるよ and せっかく来てくれたからご飯をおごるよ differ only in that the first is a 'reward' to the other person for making a special trip; the second is a decision to take advantage of (i.e., not waste) the effort that has been made.

わざわざ美容院に行ったのに、雨で髪がくしゃくしゃになっちゃった and せっかく美容院に行ったのに、雨で髪がくしゃくしゃになっちゃった are very similar. The first suggests that a lot of effort was made but the result turned out no good. The second suggests that a lot of effort was made and it was a shame that it was wasted. (It might be slightly more natural to say for the second one: せっかく美容院に行ったのに、閉まっていた.) In most cases, however, the usages are quite different.

Sekkaku: (adv) sekkaku no kōi o mu ni suru 'to bring to naught goodwill that is not come by every day' or 'to bring to naught goodwill that one should be grateful for' ('sekkaku' is not easy to translate here -- it doesn't mean 'goodwill that someone has gone out of their way to present'; it expresses regret at a wasted opportunity), sekkaku kita no ni rusu datta 'Even though I made a special trip (to see you) you were out', sekkaku desu ga o-kotowari-shimasu 'I appreciate your making this request/proposal, but I regret that I have to turn it down'.

I think わざわざ is very different to せっかく. わざわざ is closely related to わざと, which means "on purpose". I think that "purposely", "explicitly", "expressly", or "specially" might be good literal translations for わざわざ.

Japanese Q&A at Space ALC on the difference between expressions with せっかく and わざわざ:

せっかく: 後に続く事柄が、望ましいものとして評価されている。"The matter which continues afterwards is valued as something desirable."(I think that for せっかく～のに…, "I wish that..."/"it's disappointing that..." might often work and "may as well..."/"why don't [we/you/I]..." might often work with せっかく～から….)

わざわざ: 後に続く事柄が、困難・労力を伴うものであり、本来する必要がないと考えられている。"The matter which continues afterwards is something that goes along with some difficulty/distress or labor/effort/trouble, and can be thought that there isn't a fundamental need to do it."

An example with （わざわざ／せっかく）～のに…:

...来てもらったのに留守にしていて悪かったね。
It was bad that although (you) came that (he/she) isn't home.

せっかく: The speaker emphasizing that it would be desirable that that person be home ("It's disappointing that he/she isn't home given that you've come.")

わざわざ: The speaker emphasizing the difficulty/effort that it took to get there while it wasn't really necessary ("it's a pity he/she isn't home, I recognize the effort you made in coming.")

Because of the above points, せっかく and わざわざ are not interchangeable in the following sentences:

せっかく鎌倉に来たのだから、長谷まで足を伸ばそう。
"We've gone as far as Kamakura, so we may as well go a little further to Hase."

せっかくですから、遠慮なくいただきます。
"You've gone that far, (you) may as well eat up without reservation."

彼がわざわざ出向くからには何か理由があるはずだ。
"There has to be a reason why he went to the explicit trouble/effort of going out."

I've found せっかくの and are sometimes difficult to translate into English, but "precious" and "...that took great effort" (as general terms), "long awaited" and "much awaited" (for holidays and describing releases of products in advertising etc) and "kind" (for invitations etc) seem to work in a lot of cases. The Kodansha Furigana dictionary defines せっかくの when turning something down as "precious but not taken advantage of":

せっかくの休みなんだから、どこかに出掛けよう。
"It's the long-awaited holiday, so let's go out somewhere."

折角のお招きですが、今回はお受けできません。
"It's a kind invitation, but I can't accept this time."

For わざわざの I think that "expressly"/"express" etc work a fair amount of the time:

It's quite easy: use わざわざ when thanking somebody for going out of their way to do something for you, and せっかく when you want to whine that someone has (or, more commonly, you have) gone out of their way to do something for somebody else and hasn't received what you feel is sufficient recognition, recompense, or treatment. Most commonly found as "せっかく XXX のに．．．" spoken in a high, grating pitch with a pout at the end for effect. I never say this. I often have to fight the urge to slap full-grown adults who do.